This invention relates to an improved collector grid for CdS/Cu.sub.x S type photovoltaic cells.
It is known that poor design of the collector grid of a solar cell can cause significant increases in the total series resistance of the cell thus resulting in lower solar energy conversion efficiencies. Accordingly, various grid structures and designs have been tried over the years in an effort to improve the cell efficiency.
The transparent current collecting electrode, which is necesssary for the front wall type solar cells, has essentially two requirements. First, it must admit as much light to the active region of the cell as possible and second, the generated current must be collected and carried to a contact tab. In cadmium sulfide-copper sulfide type solar cells (or as more broadly designated, CdS/Cu.sub.x S type cells, x may vary between one and two, preferably closer to two) the copper compound generally has a higher resistance than is true for other type solar cells. Accordingly, relatively close grid wire spacings have been used. Close spacing was also thought necessary to minimize losses due to the recombination of carriers in the Cu.sub.x S layer.
Historically, metal mesh grids were applied to a CdS/Cu.sub.x S cell by Griffin at Harshaw Company using a mesh of approximately seventy lines per inch and having an 85% light transmission efficiency. This work is described in a report entitled "Research on Solar Energy Conversion Employing Photovoltaic Properties of CdS" Harshaw Chemical Co., AF 33 (616)-7528 Final Report Dec. 1961. Before metal mesh grids were used large area cell efficiencies were generally limited by high series resistance. The cell output efficiency was considerably improved by Clevite Corporation who determined that by switching from a grid with a spacing of 60.times.60 lines per inch and a 1 mil line width, which provided an 85% light transmission to a grid of 60.times.10 lines per inch (1pi) with 1 mil line widths, a 91% theoretical light transmission was achieved. Later work was done to optimize the collector grid design. This work was described in a report "Development of Improved CdS Solar Cells", Clevite Corp., NAS 3-13467, Final Report 1970. In this work different grid spacings between 50 and 80 lines per inch were used maintaining light transmission constant. The conclusion was that there was a trend toward increasing series resistance that would be detrimental to increasing grid spacing any further.
The metal mesh rid collector is generally photo etched from a specially rolled sheet of thin copper. Unfortunately, such copper is not only difficult to handle but also extremely costly because of its extreme thinness. Also, because it is such a delicate thinness, it is difficult to make grids and costs, in large scale manufacture, remain prohibitively high and preclude large scale utilization of solar cells for low cost terrestial use.
In most cells of the prior art, in order to achieve an optimized grid collector, the grid wires must be spaced closely enough to efficiently collect the generated current as noted above and at the same time space widely enough to provide sufficient transmission of light to the active region of the cells.
The compromises required in the design geometry of the present grid collectors generally result in an optimized maximum light transmission of approximately 80 to 90%, generally centering around 85%. This in itself results in lower efficiencies than would otherwise be desirable. But these requirements are contradictory. Accordingly, the use of optimum spacing and geometry of the collector grid wire for the best cell performance is an object of this invention.
A further object of this invention is to provide improved collector grids for CdS/Cu.sub.x S solar cells which provides a more easily manufactured grid.
Another object of this invention is to provide an improved lower cost grid for CdS/Cu.sub.x S solar cells which provides a more easily manufactured grid.
Still another object of this invention is to provide an improved collector grid for CdS/Cu.sub.x S solar cells which provides an improved cell output efficiency.